Wireless communications systems are frequently implemented as one or more communications cells. Each cell normally includes a base station which supports communications with end nodes, e.g., wireless terminals such as mobile nodes, that are located in, or enter, the communications range of the cell's base station. Signals transmitted between a base station and a mobile node may be transmitted in two possible directions, e.g., from the base station to the mobile node or from the mobile node to the base station. Transmission of signals from the base station to the mobile is often called a downlink. In contrast, transmission from the mobile to the base station is commonly referred to as an uplink. Communication cells are subdivided into sectors in some systems. Within a cell or a sector of a cell, the unit of communications resource is a symbol, e.g., QPSK or QAM symbol. In the case of an orthogonal frequency division multiplexed (OFDM) system a symbol may be transmitted on a frequency tone (e.g., subcarrier frequency) for one time slot. The total available communication resource, which tends to be limited, is divided into a number of such symbols (units) which can be used for communicating control and data information between a base station and one or more mobile nodes in the cell. For transmission purposes, the subcarrier frequencies are modulated on a carrier frequency. The carrier frequency and associated bandwidth encompassing the range of subcarrier frequencies may be reused in sectors and cells.
Bandwidth reuse is an important method of improving spectral efficiency in a cellular communication system. In particular, many cellular systems use special technology, such as spread spectrum technology, to allow the reuse of the same spectrum in multiple cells. This reuse of the same spectrum in multiple cells can result in operational problems at or near the cell boundaries. FIG. 1 shows a cellular system 100 utilizing the same spectrum in adjacent cells. In FIG. 1, a first cell 106 represents an area of coverage 110 in which a first base station, base station 1 102, may communicate with wireless terminals. A second cell 108 represents an area of coverage 112 in which a second base station, base station 2 104 may communicate with wireless terminals. Cells 106 and 108 are neighboring cells which share a common boundary. In cellular communications systems, there are certain boundary areas where the signal strengths, e.g., measured in terms of pilot power, received from different base stations are almost equally strong (sometimes referred to as 0 dB regions) and these areas are treated as the “boundary region” or ‘boundary’ between cells. In FIG. 1, the coverage area 110 for base station 1 102 and the coverage area 112 for adjacent base station 2 104 overlap and create a boundary region 114.
Consider an exemplary case, in which a wireless terminal is located in the cell boundary region 114. The wireless terminal can be fixed or mobile. When the wireless terminal is communicating with one of the base stations, e.g. base station 1 102, in the boundary region 114 the interference from the other base station, e.g. base station 2 104, may be almost as strong as the signal from the serving base station 102. Indeed, due to fading and other impairments in the wireless channel, the signal may be much weaker than the interference from time to time. Therefore, the connection for that wireless terminal may not be robust in such a case. The signal reliability in the boundary region 114 may be low and generally the power has to be boosted to overcome the noise. A weak signal with low reliability may result in loss of or disruption of communications for the user of the wireless terminal resulting in customer dissatisfaction. Many wireless terminals are mobile devices operating on limited battery resources; therefore, any additional expenditure of power required by the mobile can be very significant, as it will directly reduce the user's operational time between battery recharge or replacement. In addition, the cost to serve that wireless terminal in boundary region 114, in terms of power and bandwidth allocation in the serving base station 102, may be relatively high. Hence, there is a need for apparatus and methods to improve the service in the cell boundary region 114.
Some cellular systems using special technology, such as spread spectrum technology, also subdivide the cells into sectors and allow the reuse of the same spectrum in all the sectors. This reuse of the same spectrum in all sectors of a cell can result in operational problems at or near the sector boundaries in addition to the above discussed cell boundary problems. The sector boundary region problems encountered are very similar or identical to the cell boundary regions problems. Hence, there is also a need for apparatus and methods to improve the service in the sector boundary regions.